The invention concerns flat-sheet membrane separators, especially as used in wastewater treatment or biosolids waste management systems. Specifically the invention encompasses an improved, more efficient arrangement of flat-sheet membrane assemblies as process cells in a tank, along with positioning of liquid piping and air conduits for space saving, energy saving and higher efficiency operation.
Flat-sheet membrane technology has some inherent advantages over alternate technologies when used in submerged membrane bioreactor (SMBR) applications. In particular, flat-sheet technologies are generally considered easier to operate and more robust than hollow-fiber or tubular membrane systems. However, flat-sheet technology is generally less space and energy efficient than competing technologies due to low packing density and longer hydraulic residence times (HRT).
Packing density is a measurement of space efficiency and is typically calculated by dividing membrane area by the volume occupied by membrane equipment. There are over 2,000 SMBR systems worldwide using flat-sheet membranes with packing densities on the order of 10-60 m2/m3. Competing technologies generally report packing densities between 150-300 m2/m3. Packing density does not necessarily correlate to capacity, as filtration rate can vary significantly among available technologies depending on flux.
Flux is a unit of velocity that equates to the rate at which water can be filtered for a given membrane area. Accounting for differences in flux capabilities and additional installation space requirements (piping, etc.), volumetric space requirements for a given treatment capacity can be calculated in terms of HRT (hydraulic residence time).
HRT is equivalent to the amount of time spent by a unit volume of water in a reactor during treatment and is the best measure of space efficiency. HRT numbers generally range between 1.0 hr to 4.0 hr for flat-sheet technologies and between 0.4 hr to 1.0 hr for hollow fiber systems. There are two main reasons for the high HRT reported for SMBR systems using flat-sheet technology: inefficient structural design (primarily channels, piping and covers) and space requirements necessary for the development of so called roll patterns induced by scour air flow.
The method of piping and air scouring described herein decreases HRT for SMBR systems using any membrane geometry but has the most benefit for flat-sheet technology. HRT can be decreased by 40% or more. In addition, the invention eliminates dimensional constraints on reactor design in terms of length to width aspect ratio, increases energy efficiency and reduces construction cost.
In SMBRs (also referred to as immersed or submerged membrane bioreactors) a combination of mixed liquor (activated sludge and wastewater) and air moving tangentially across the surface of membranes (cross-flow) scours filtered solids from the surface of the membrane. It is this scouring, cleaning action that makes submerged membrane technologies commercially viable.
It is generally accepted that superficial (tangential) velocities of approximately 0.5 ft/s to 1.5 ft/sec are required to promote slug-flow. Slug-flow is a hydrodynamic condition that research suggests promotes the most efficient air scouring in submerged membrane applications. To achieve this kind of velocity, bubbles are introduced at the bottom of membranes arranged in various geometries to create a pumping effect. The rise velocities of bubbles are generally at least 10 times typical mechanical pumping rates and are sufficient to induce the desired slug flow regime. Once the bubbles rise to the water surface, sufficient depth and a viable flow path are necessary to allow the development of a roll pattern where filtered mixed liquor is circulated back down to the diffusers located at the bottom of the tank.
For flat-sheet systems it is common practice for discrete assemblies of membrane equipment (sometimes referred to as membrane units, racks or cassettes) to be supplied with dedicated submerged air diffusers that induce a circular flow pattern around each assembly with an upward velocity on the order of 0.5-1.5 ft/sec (see FIG. 1). In this configuration, the rising two-phase flow (bubbles and mixed liquor) can flow back down the sides of each individual assembly to be re-entrained at the bottom of the assembly and sent back up through the sheets in a circular fashion (called a roll pattern). The roll of combined mixed liquor and air can also be described as an internal recycle (IR) stream. This design practice has been effective for decades but adds cost, increases the overall volume requirements and limits energy reduction options.
Putting empty space between membrane equipment decreases space efficiency but is currently viewed as necessary by purveyors of flat-sheet membrane equipment. The problem (of reduced space efficiency) is further compounded by fluid conveyance requirements, inefficient piping design and the need for reactor covers.
In a SMBR system there are basically three fluids that must be conveyed into and out of reactors: scouring air, filtered water (permeate) and mixed liquor. Common practice is to separate all of these conveyance systems and build them into reactors using dedicated space for access, increasing cost and decreasing packing density. In large plants, concrete walls are generally poured to support piping systems and tank covers. Walls are also necessary to separate membrane equipment into process zones with specific and limited length to width aspect ratios. In addition to concrete walls, concrete channels are built to feed and return mixed liquor from membrane zones (tanks).
In FIG. 2 a diagram of a typical SMBR reactor is shown in plan view. Per the diagram mixed liquor feed and return channels are located on opposite sides (ends) of the tank (one inlet and one outlet). Channels are generally formed from concrete and connected to a reactor (tank volume) by gates or simple wall cutouts. Permeate and air pipes run adjacent to the submerged membrane assemblies and are mounted to the tank walls using brackets or other methods. In many cases the membrane assemblies are stacked two high (also called double-deck configuration) to increase packing density and reduce HRT. However, stacking membrane equipment has several issues including access to lower membrane elements, additional piping, the development of dissolved oxygen gradients from the water surface down to the tank floor and increasing wall thickness to accommodate taller walls (driving up concrete costs and carbon footprint). Moreover, double-deck configurations do not cut air requirements in half as expected. At increased water depths air scouring (volumetric) requirements increase to achieve the same bubble rise rate and the added discharge pressure (given increased water depth) drives up energy requirements to produce a given volume of air.
Air bubbles emitted at or near the bottom of the tank transfer oxygen as they rise to the surface. The filtered (thickened) aerated mixed liquor is subsequently depleted of oxygen as it returns to the diffuser down the side of the membrane assembly, creating an oxygen gradient. Dissolved oxygen (DO) concentration gradients can degrade SMBR performance in two ways. First, without sufficient DO, biological process can be inhibited, impacting effluent quality. Second, in deeper tanks DO can be low enough (near zero) to lead to anaerobic deteriorating sludge quality by allowing the formation of extracellular polymeric substances (EPS), the leading type of organic foulant. Arranging single-deck or double-deck membrane assemblies in rows with a single point of feed and a single outlet can lead to significant concentration gradients that can impact system performance. In FIG. 15, typical numbers are given for MLSS concentration as a function of aspect ratio (length to width). This phenomenon limits the allowable length to width ratio of tanks. The varying concentration of mixed liquor coupled with the changing concentration in pollutants from the tank inlet to outlet points theoretically creates small reactors (one per filter assembly) operating in series and reducing treatment efficiency. Methods have been proposed to prevent this condition, including limiting the length to width ratio (3:1 is typical); using pipes located beneath membrane assemblies to feed and return mixed liquor; using internal pumps to keep tank contents well mixed; or having multiple feed return points in channels. However, none of these methods will completely eliminate thickening gradients, as mixed liquor thickened by membranes adjacent the tank inlet will be filtered again (further thickened) if separated from the tank outlet by another membrane assembly. Also, many methods used to minimize MLSS gradients create a separate problem called short-circuiting whereby pollutants are filtered prematurely before biological processes can take place. Finally, covers are often necessary and mechanical support structures added, taking additional space and increasing cost.
As noted above, current art is space-consuming and inefficient. The objective of the invention is to greatly increase efficiency in SMBR systems and improve performance by eliminating MLSS and DO gradients irrespective of tank aspect ratio.